1. Field of the Invention
The present invention relates to a hybrid printed circuit board, which has circuit conductors to pass small signal currents and circuit conductors for large currents and which is incorporated in electric junction boxes used in automotive wiring harnesses.
2. Description of the Prior Art
Hybrid printed circuit boards having power circuits and signal circuits are generally made by bonding a thin copper foil for signal circuits to one side of an insulating substrate and a thick copper foil for power circuits to the other side and then etching these foils using mask patterns. In such printed circuit boards, because the thickness of the copper foil is limited by the depth that can be penetrated by etching, it is necessary to increase the conductor width in order to increase the current carrying capacity of the power circuit. This is turn makes it difficult to reduce the size of the circuit. Forming a through-hole in the power circuit conductors involves the step of forming a conductive layer on the inner surface of a hole piercing through the insulating substrate, as with the ordinary signal circuit conductors. With such through-holes, it is difficult to increase the thickness of the conductive layer, so that large currents cannot be carried.
To cope with this problem, a hybrid printed circuit board as shown in FIGS. 16 through 18 has been proposed (Japanese Patent Preliminary Publication No. 63-237495). In this proposed hybrid printed circuit board with signal circuit conductors b and power circuit conductors c formed on an insulating substrate a, the signal circuit conductors b are made by pattern-etching a copper foil adhering to the substrate a and the power circuit conductors c are formed by fixing a desired pattern of a punched conductive metal sheet to the substrate a. Further, the through-hole portion c1 of the power circuit conductor c is formed by inserting a cylindrical projection c2 drawn from the circuit conductor c into the hole in the insulating substrate a.
This hybrid printed circuit board, too, requires a number of soldering steps, a proper material for soldering and sufficient heat resistance because terminals are soldered to the signal and power lead-out through-holes in the substrate.